1. Field of the Invention
The invention relates to filter canister for gas masks. In one aspect, the invention relates to a method of filling a filter canister with a particulate filter material. In another of its aspects, the invention relates to filling a filter cartridge having a concave profile conforming to the curvature of the wearer's face. In yet another of its aspects, the invention relates to a filter canister for a gas mask wherein the canister has an oblong shape and a concave profile for conforming to the shape of a user's face.
2. Description of the Related Art
PCT Application No. US01/12545, published Oct. 25, 2001, discloses a bayonet-type connector for connecting a removable filter to a gas mask. A filter canister having an oval shape is also disclosed, comprising parallel planar inlet and outlet faces separated by a perimeter wall.
Conventional replaceable circular or elliptical gas mask filters with parallel planar inlet and outlet faces can be mounted to both sides of the mask to extend laterally outwardly of the mask. Such filters typically comprise a folded paper or fabric particulate filter and a granulated carbon adsorption filter. Settling of adsorbent in gas mask filters before or during use can result in localized areas in which the gas flow path through the adsorbent is shorter than through the bulk of the adsorbent. This results in increased air flow through a smaller volume of adsorbent with a consequent early breakthrough of contaminant material. In order to maximize the density of the carbon granules and avoid settling and the introduction of uncontrolled flow channels through the adsorption filter, the adsorbent (i.e. granulated carbon) is typically placed in the filter canister through a process of “raining” or “snow storm” filling the granules into the filter in such a way that the density of the adsorbent is maximized. In this method, the adsorbent drops through a tube containing four to five metal wire screens with the same face area as the container to be filled. The screen opening size and spacing is related to the particle size of the adsorbent being filled. No single particle has the opportunity to pass through the tube without hitting the screen wires. This effectively randomizes and uniformly distributes the particles across the surface of the container being filled. The adsorbent is also structurally constrained with a compressive force. The “snow storm filling” process requires that the inlet and outlet faces be planar.
Filters having a concave profile to conform closely to the curvature of the wearer's face offer several advantages over conventional filters having planar inlet and outlet faces. However, the curvature of the filter is not conducive to the conventional “snow storm filling” method of placing the adsorbent in the filter canister, and can prevent the adsorbent from assuming a minimum required density. The “snow storm filling” method is only applicable to an adsorbent bed which has a uniform flat depth. The “snow storm filling” method is not effective for a conformal filter design having a carbon filter bed that has an arcuate contour on the inlet and outlet faces with a constant bed depth between them. In order to ensure a minimum required thickness of the adsorbent at a minimum required density, a greater thickness of adsorbent may be needed as compared to a filter having planar inlet and outlet faces, increasing the cost of the filter.